Orbiting and geosynchronous satellites are in wide use for communication between ground communications hubs and directly between communication users. The revenue of the satellite industry exceeded $200 B in 2014, and over 400 satellites are currently in geosynchronous orbits.
The preparation and delivery costs of such satellites represents a considerable expense, however, and rocket launches themselves represent a non-negligible risk. While satellites have the advantage of requiring no significant energy in order to remain in orbit for years, they lack the ability to recover and to change orbits as may be required to modify the communication equipment or the serviced area. Additionally, while the satellites offer wide area coverage their great communications distance results in the need to use high transmission power and relatively narrow antenna apertures, which in turn results in communications time delays.
The idea of sub-orbital vehicles as airborne communications nodes utilizing both lighter than air and heavier than air vehicles is well known. For example, the military use communication relay aircraft in specific applications where they offer a versatility advantage compared to satellites.
Because of their required high launch energy and extreme system reliability, satellites present substantially higher initial costs than aircraft. However, by maintaining orbit for extended periods of time they can offer lower cost per hour. In order to compete effectively with satellites, an aircraft needs to provide large communications area coverage, dependable service, and many hours of operations per launch and recovery cycle. To provide large area coverage for high frequency band line-of-sight communications and for dependable service, such an aircraft must fly at high altitudes and above weather. For very long endurance the aircraft will preferably be unmanned and use a renewable energy source. When flying above the weather, solar energy is a readily available energy source for aircraft with endurance greater than a few days. Solar powered unmanned aircraft first flew in 1974, and manned aircraft first flew in 1979. Solar energy, however, is a relatively diffuse source that can limit the power available to propel the aircraft. In addition, reliance on solar power can restrict the latitude and/or time of year that such aircraft can operate effectively.
Flying at high altitude at low power levels requires a very light aircraft with a large lifting surface area (i.e. low wing loading). Unmanned solar powered aircraft developed between 1983 and 2003 by AeroVironment, Inc. under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) program aimed at ever increasing aircraft cruise altitude. On Aug. 14, 2001 their Helios unmanned aircraft set an altitude record of 96,863 feet (29,524 m). To achieve such a high altitude the highly innovative Helios design included a wing span of 247 feet, a wing area of 1,976 square feet, and a normal weight of only 1,600 lb (0.81 lb/ft2 wing loading). In order to achieve such a low weight, Helios used an all-wing design, a very light flexible structure, a high level of weight distribution along the span (span-loading), and a distributed powerplant configuration. Unfortunately, such extremely flexible structures are relatively fragile. The Helios was lost in an inflight failure, with aircraft loss due to wind gusts on Jun. 26, 2003.
Flying in clear weather or at low altitude is less challenging, while less useful for communications purposes, has provided progress in developing the system reliability required for long endurance flight. For example, QinetiQ's 74 ft span Zephyr unmanned solar powered aircraft set, on July 2010, a 336 hour, 22 minute endurance record for unmanned aircraft. While the Zephyr endurance record was set during summer months (from July 9 to July 23 in Yuma, Ariz., 32 degrees north latitude)), in later tests Zephyr flew 11 days in winter conditions.
Long duration solar powered aircraft are also hampered by the relatively low energy density of regenerative batteries and regenerative fuel cells required to power the craft at night. Flying at high altitude is additionally challenging in this regard, as the low air density increases the power required for propeller driven aircraft (e.g. by a factor of 3× relative to sea level at 56,000 ft and a factor of 4× relative to sea level at 69,000 ft). Additionally, the increased dynamic viscosity at high altitude (resulting in reduced Reynolds numbers) results in reduced aerodynamic performance in terms of aircraft lift/drag ratio and propeller efficiency for slow flying aircraft.
We assume that in order for the high altitude solar powered unmanned aircraft to become a desirable alternative to the low-orbit satellite and to capture a portion of the large communication market it needs to offer:                a. a network of hundreds of aircraft on station worldwide providing continuous coverage        b. an airframe of adequate ruggedness to withstand climb and descent in less than ideal weather        c. a flight safety track-record documenting an ability to sustain approximately 9,000 flight hours per year per aircraft, using aircraft certified by the Federal Aviation Administration (FAA) and of other world agencies and the approval of the Air Traffic Control (ATC)        d. sufficient geographic coverage during winter        e. a cost per communication data rate and per area coverage that is reasonably competitive with geosynchronous satellites.Providing such a combination of essential aircraft attributes requires that a large and very light aircraft achieve a safety record comparable to that of current commercial airlines. While many individual technology advancements (e.g. improved battery energy density and/or solar cell efficiency) can contribute to this, it is important to combine the performance of all contributors.        
A key challenge for a solar powered aircraft to provide an acceptable market entry is the ability of the aircraft to collect solar energy at low sun angles, such as in winter and early and late in the day. Prior art aircraft configurations typically utilize a flat, stretched wing, which offers high aircraft lift/drag (L/D) ratio and low power requirements. The challenge of relying on solar energy is generally addressed by: a) building ultra-light, low wing loading airframes that are not capable of surviving gusts, b) flying in mid-summer (when high sun angle provides optimal solar cell performance), c) flying in geographic locations where sun angle is high (e.g., low latitudes), d) flying at low altitude and in good weather (permitting lower speed and reduced power requirements), e) not flying through the night, and f) gliding at night using altitude gained during the day (with resulting loss of altitude), and frequently utilize a combination of these. Such approaches, however, are not viable for the intended use of replacing orbiting communications satellites.
The challenge of flight through the night can be partially addressed by use of high energy density batteries (for example, Lithium-Sulfur batteries) with energy densities of up to 220 Wh/lb. Laboratory research indicates further improvement in performance of several chemistries of rechargeable batteries are possible.
Some solar powered aircraft have attempted to address the problem of generating adequate power at low sun angles by presenting solar cells at angles that are more vertical relative to the plane of the aircraft when in level flight. U.S. Pat. No. 6,931,247 (to Cox and Swanson) discloses a lightweight solar powered aircraft configured as a flying wing, constructed as a transparent film over a lightweight frame. The wing is divided in segments that are joined by hinges, with each segment carrying motor/propeller assemblies. The hinges permit adjustment of the dihedral angles of the segments of the wing that aid in orienting solar cells mounted on the wing toward the horizon during flight. Hinge placement and orientation permit various wing configurations, including “M” and “W” configurations with both positive and negative dihedral angles. All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. United States Patent Application Publication No. 2010/0213309 (to Parks) shows a similar lightweight aircraft, which includes tail booms that carry additional solar cells and wing-mounted pylons that position motor/propeller assemblies both above and below the wing. Differential thrust applied by these motor/propeller assemblies is used to adjust the pitch of the aircraft for altitude adjustment. The taught aircraft, however, utilize motive power (in the form of motor/propeller assemblies) in each wing segment, apparently as a design constraint necessitated by flexibility of the taught airframes. This increases expense and design complexity, and provides numerous opportunities for component failure.
Another approach that has been attempted to improve the efficiency of solar cell performance in aircraft flying at low sun angles is to provide a vertical surface on which solar cells are mounted. In such designs, however, the aircraft L/D ratio is necessarily reduced in proportion to the improved low sun angle collection. This results from the additional parasitic drag of vertical surfaces beyond those necessary for efficient flight, resulting in increased power requirements. In addition, such vertical surfaces can negatively impact aircraft stability in inclement weather.
Still another approach that has been attempted in the prior art is to utilize a flexible wing surface that carries solar cells. When the central portion of the wing is weighted (for example, through carrying batteries or payload) the downwards inflection of the flexible wing produces a curve that angles a portion of the solar cells towards the horizon. An example of such a design is the aforementioned Helios aircraft. Bending of the aircraft's flexible wing combined with a built-in 5 degrees dihedral of the outboard wing sections provided adequate solar energy collection during flight at the relatively high sun angles provided by performing the flight test in August at a low latitude (i.e. Kauai, which lies at 22° N latitude). Unfortunately, as demonstrated by the loss of Helios aircraft due to weather, such a flexible wing does not provide an aircraft that is sufficiently rugged for long duration flight.
Thus, there is still a need for solar powered aircraft that can provide consistent long duration flight times at high altitudes and high latitudes.